A diet including unlimited amounts of junk food can cause rats to become so addicted to the unhealthy diet that they will starve themselves rather than go back to eating healthy food, researchers have discovered.

In a series of studies conducted over the course of three years and published in the journal Nature Neuroscience, Scripps Florida scientists Paul Johnson and Paul Kenny have shown that rats’ response to unlimited junk food closely parallels well-known patterns of drug addiction -- even down to the changes in brain chemistry.

"What we have are these core features of addiction, and these animals are hitting each one of these features," Kenny said.

In their first study, the researchers fed rats on either a balanced diet or on the same diet plus unlimited access to junk foods purchased at a local supermarket, including processed meats and cakes. Within a short time period, the rats on the junk food diet began to eat compulsively and quickly became overweight.

"They’re taking in twice the amount of calories as the control rats," Kenny said.

The researchers hypothesized that the rats were eating compulsively because, like drug addicts, they had become desensitized to smaller amounts and needed more and more for the same rush of pleasure.

Many recreational drugs work by directly stimulating the brain’s pleasure centers, particularly the dopamine receptor known as D2. Overstimulation of this receptor causes the body to start producing less dopamine, leading the addict to compensate by taking more of the drug.

Since dopamine can also be released by pleasurable activities such as food or sex, Kenny and Johnson speculated that food addiction could develop in the same way. To test whether the rats had, in fact, become habituated to dopamine, the researchers took the rats from the first experiment and hooked their brains up to a device that would directly stimulate their D2 receptors when they ran on a wheel.

Rats eating a junk food diet ran on the wheel significantly longer than rats fed a normal diet, suggesting that their receptors had indeed become desensitized. This "profound" desensitization occurred after just five days on a junk food diet.

"They’re not experiencing rewards the way they should," Kenny said. "When you experience that, one way of feeling better is to go back to the junk food."

"They lose control. This is the hallmark of addiction."

In another test of their addiction hypothesis, the researchers used a virus to block the D2 receptors in healthy rats. All those rats soon became compulsive eaters.

"This is the most complete evidence to date that suggests obesity and drug addiction have common neurobiological underpinnings," Johnson said.

Having established that the junk food rats had become addicted, Johnson and Kenny wanted to know how far this addiction would push them. So they took both junk-food addicted rats and rats that had not previously been exposed to such food, and exposed them to electrical shocks whenever they ate junk. Rats that had just been introduced to junk food quickly stopped eating it, while the addicted rats ignored the discomfort and kept eating.

Perhaps the most shocking finding came when the researchers took away the addicted rats’ access to junk food and started feeding them only healthy rat chow again -- the same diets the rats had eaten as pups. When junk food was no longer available, the rats simply refused to eat for two weeks.

"They actually voluntarily starved themselves," Kenny said.

"It’s almost as if you break these things, it’s very, very hard to go back to the way things were before. Their dietary preferences are dramatically shifted."

The research strongly suggests that many modern humans also suffer from junk food addictions. Kenny notes, however, that unlike the rats, all humans with access to junk food do not become obese. He attributes this difference to the influence of health knowledge and social pressure in moderating people’s natural eating habits.

"The rats don’t suffer from the same social pressures that we do," he said.

The idea of junk food addiction is not a new one, and the dopamine-junk food connection was actually put forward by former FDA Commissioner David Kessler in his best-selling book, The End of Overeating.

"Certainly, we see this addictive pattern in humans," nutritionist Sandy Livingston said. "They know they shouldn’t overeat, but they do it anyway."

Livingston expressed hope that better knowledge about the biochemical side of food addiction might result in lessened guilt and judgment surrounding obesity.

"A lot of people blame themselves -- ’Why don’t I have any willpower?’" she said.

"Food can be highly addictive," said author and nutritional supplement producer Jordan Rubin. "When people describe overeating and weight loss as a battle, this is why."

He called for more research into which individual components of junk food, such as MSG, might be behind its addictive effects.

Obesity researcher Ralph DiLeone of Yale University noted that more research is needed into the long-term effects of such addiction, even if an animal later switches its diet and loses weight.

"They might be a normal weight, but how they respond to food in the future may be permanently altered," he said.

Suggesting yet another area for future research, Kenny has expressed hope that better biochemical understanding of food addiction might someday enable the development of a drug or vaccine as a treatment for compulsive eating.

Johnson and Kenny’s research was funded by grants from the National Institutes of Health, Bank of America and The Margaret Q. Landenberger Research Foundation.

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